In a conventional image-forming apparatus, an electrostatic latent image, which is formed on a photoconductor drum and to which toner adheres, is developed, and the resulting toner image is copied onto a copying material that has been wrapped onto a transfer drum.
For example, as illustrated in FIG. 27, such an image-forming apparatus has corona charger 102 for attracting a copying material P and corona charger 104 for transferring a toner image formed on the surface of a photoconductor drum 103 onto the copying material P, which are separately installed inside a cylinder 101 having a dielectric layer 101a. Thus, the attracting process and the copying process for the copying material P are carried out separately by the respective chargers 102 and 104.
Further, as illustrated in FIG. 28, some image-forming apparatuses are provided with a cylinder 201 having a two-layer structure of a semiconductive layer 201a (an outer layer) and a base layer 201b (an inner layer) and a grip mechanism 202 for holding the transported copying material P along the cylinder 201. In this type of image-forming apparatus, the grip mechanism 202 grips the transported copying material P at its end and wraps it along the surface of the cylinder 201, and then the surface of the cylinder 201 is charged by applying a voltage to the semiconductive layer 201a forming the outer layer of the cylinder 201 or allowing the charger installed inside the cylinder 201 to discharge so that the toner image on the photoconductor drum 103 is copied onto the copying material P.
In the image-forming apparatus as shown in FIG. 27, however, since the cylinder 101 serving as a transfer roller has a one-layer structure with only the dielectric layer 101a, it needs to have the above-mentioned corona chargers 102 and 104 installed inside thereof. This limits the size of the cylinder 101, and the resulting problem is that it is difficult to make the apparatus compact.
Moreover, in the image-forming apparatus as shown in FIG. 28, since the cylinder 201 serving as a transfer roller is designed to have the two-layer structure so that it is readily charged so as to copy the toner image onto the copying material P, it is possible to reduce the number of chargers. However, the application of the grip mechanism 202 makes the entire construction of the image-forming apparatus more complex, resulting in an increase in the number of parts in the entire apparatus and an increase in the manufacturing costs of the apparatus.
In order to solve the above-mentioned problem, for example, Japanese Laid-Open Patent Publication No. 74975/1990 (Tokukaihei 2-74975) discloses an image-forming apparatus wherein: a transfer drum, constituted by conductive rubber and a dielectric film that are laminated on a metal roll that is grounded, is provided and a corona charger, which is driven by a unipolar power supply, is installed in the vicinity of the separation position of the copying material associated with the transfer drum. In this image-forming apparatus, an electric charge is induced on the dielectric film by the corona charger so as to allow the transfer drum to attract the copying material. Here, as the copying material is attracted, more electric charge is induced, making it possible to carry out a copying process.
Therefore, with the above-mentioned image-forming apparatus, since a single charger charges the surface of the transfer drum, allows the copying material to be attracted, and carries out the copying process, it is possible to make the transfer drum compact. Further, without the need for the grip mechanism 202 or other devices for holding the copying material, it is possible to attract the copying material by using a simple structure.
Moreover, Japanese Laid-Open Patent Publication No. 173435/1993 (Tokukaihei 5-173435) and U.S. Pat. No. 5390012 disclose copying apparatuses in which: a transfer drum having at least an elastic layer made of a foamed material and a dielectric layer covering the elastic layer is provided and toner images with respective colors successively formed on the photoconductive drum are successively overlapped and transferred onto a copying material that has been attracted onto the transfer drum so that a color image is formed on the copying material.
In these copying apparatuses, with respect to a method for holding the copying material on the transfer drum, the copying material is electrostatically attracted onto the transfer drum by using an attracting roller that serves as an electric-charge applying means. Moreover, between the elastic layer and the dielectric layer in the transfer drum, a clearance not less than 10 .mu.m is provided so that electric charge is accumulated on the back surface (the side not having the copying material) of the dielectric layer so as to make a construction in which electric potential is maintained without being affected by environmental conditions. Thus, the attracting capability, that is, attracting property of the copying material, is improved.
Furthermore, Japanese Examined Patent Publication No. 84902/1993 (Tokukouhei 5-84902) discloses a multiple copying apparatus having a construction which is provided with a transfer drum for transferring a toner image formed on the photoconductor drum onto a copying material at a transferring position. On the transfer drum, a dielectric layer with a thickness of 70 to 200 .mu.m, which has a dielectric constant of 3.0 to 13.0 and a critical surface tension of not more than 40 dyne/cm, is stacked. The electric characteristics of such a dielectric layer make it possible to maintain transferring performances even under severe environmental and ambient conditions, and the critical surface tension ensures the cleaning property of the surface of the transfer drum after separation of the copying material.
Additionally, with respect to cleaning mechanisms for an intermediate transfer belt on which a toner image, formed on the photoconductor drum, is temporarily overlapped and transferred prior to being transferred onto a copying material, for example, Japanese Laid-Open Patent Publication No. 323835/1993 (Tokukaihei 5-323835) discloses a method for combinedly using a brush and a blade, and Japanese Laid-Open Patent Publication No. 313512/1993 (Tokukaihei 5-313512) discloses a method in which after eliminating electricity from the surface of the intermediate transferring belt using a brush, the surface is cleaned by a blade.
However, since the surface of the transfer drum is charged through an atmospheric discharge by a corona charger, the construction described in Japanese Laid-Open Patent Publication No. 74975/1990 (Tokukaihei 2-74975) is susceptible to the effects of environmental conditions, such as temperatures, moisture, etc. of the air, resulting in unevenness in the surface electric potential of the transfer drum. Consequently, this construction tends to have disadvantages such as insufficient attraction of the copying material and degradation in print quality. Further, if the surface of the transfer drum has scratches, the electric-field region will become smaller upon charging by an atmospheric discharge, and the transferring electric-field balance tends to be disordered at the scratched portions. Therefore, a transferring process, if carried out in such portions, tends to cause defects in transfer such as white blanks, resulting in degradation in the picture quality.
The constructions respectively described in Japanese Laid-Open Patent Publication No. 173435/1993 (Tokukaihei 5-173435) and U.S. Pat. No. 5,390,012 have the following disadvantages: In general, it is considered that, if different kinds of copying material are used, since the amount of electric charge of the copying material is different within a given period of time (nip time) during which the copying material passes between the transfer drum and the attracting roller, the transferring electric field upon electrostatic transfer from the photoconductor drum to the copying material differs to a great degree depending on the kinds of copying material. In other words, if the nip time is set constant in any kind of copying material, since the amount of electric charge injected within the constant time differs depending on the kinds of copying material, the electrostatic transferring capability of the transfer drum is reduced depending on the kinds of copying material, with the result that electrostatic transfer occasionally is not available by the photoconductor drum. The resulting problem is that a toner image, formed on the photoconductor drum, is not properly transferred onto the copying material.
Further, if the nip time is set constant in the same manner as described above, the electrostatic attracting capability of the transfer drum is reduced depending on the kinds of copying material, resulting in failure to properly attract the copying material onto the transfer drum. Consequently, the copying material tends to peel from the transfer drum and a toner image, formed on the photoconductor drum, is not properly transferred onto the copying material.
Moreover, after separation of the copying material, the copying-material attracting charge, the counter charge of toner, etc., still exist on the surface of the transfer drum, and in the case of a strong electric constraining force of the toner adhering to the surface of the transfer drum, the cleaning property tends to be altered. The tendency is particularly strong in the case of toner with high electric resistance. When the cleaning property is altered as described above and the toner on the surface of the transfer drum becomes difficult to remove therefrom, the resulting problems are contamination of the back surface of the copying material and difficulty in carrying out the next attracting and transferring operations.
Here, from the point of view of cleaning mechanisms and static-eliminating mechanisms for the transfer drum and the intermediate transfer belt, any of the above-mentioned conventional constructions is merely a modification of a method used for the image-bearing body such as a photoconductor drum and a photoconductor belt, and fails to provide an optimum mechanism for the transfer drum. Consequently, for example, the attracting electric potential for attracting the copying material becomes unstable due to unnecessary electric charges which remain on the transfer drum after cleaning, resulting in problems, such as separation of the copying material from the dielectric layer during transportation, dislocation thereof, and the subsequent degradation in the picture quality.
The unnecessary charges after cleaning correspond to the sum of frictional electrification caused when the blade in the cleaning mechanism sweeps the surface of the transfer drum and the counter charge of toner.